Deposits occur in pipework in all fields, regardless of whether or not the liquids are water-based. When the liquid is water, specific mention may be made of problems which occur in piping potable water, industrial water, boiler water, cooling water, etc.
Deposits may be formed by calcium carbonate (scale), by metal oxides, by atmospheric dirt in circuits open to the air, by microbes, or by corrosion products. With mixtures or solutions of various substances, other types of deposit may also be encountered (e.g. due to substances precipitating from the solution).
The liquid may also attack the surface of the pipework, and one particular form of corrosion is due to metal, generally iron, being attacked by water since, thermo-dynamically speaking, there is no known domain over which water and iron can remain stably in contact under natural conditions.
These phenomena give rise to considerable drawbacks. Deposits progressively block up pipes, thereby reducing flow rates or increasing head losses, and they also reduce heat exchange capacity, whereas corrosion damages pipework and may lead to breakage.
With natural water, the practical equilibrium between calcium bicarbonate and carbon dioxide is governed by rather complex laws and a shift in the equilibrium position can give rise to chemical reactions in which calcium carbonate is dissolved (aggressiveness) or deposited (scaling), and these reactions may be superposed on the straight-forward electro-chemical corrosion reactions which are specific to metals.
Thermodynamic calculation methods have been developed for attempting to estimate the scaling or corrosive nature of a given water. However, the large number of such methods (Tillmans' method, Langeliers's method, Hoover'diagram, Hallopeau's method, Franquin and Marceaux's diagram, . . . ) is witness to the difficulty of this approach. These methods are based on studying pure solutions under determined conditions of pH, temperature, and concentration, and they are not capable of taking account of the complexity of practical situations. In addition, the results of such calculations are often of the YES/NO type as to the possibility of precipitation taking place, without giving any possibility of investigating the kinetics of the phenomena.
In order to mitigate these drawbacks, methods and apparatuses have been developed for using the water of the circuit concerned to obtain a representation (which may be accelerated) of these phenomena so as to be able to correct them and possibly prevent them from taking place.
A first method consists in placing thermocouples in a special circuit off the main circuit and in measuring variations in the heat exchange coefficient. This method gives an indication of the state of the apparatus without requiring direct inspection, e.g. in the cooling circuits of electricity power supply stations. This method thus does not make it possible to forecast scaling but only to observe it, and then only providing that the same conditions are maintained in the special circuit as in the main circuit, in particular with reference to temperature. This method is lengthy in application since the phenomenon takes place under real operating conditions and since cleaning the special circuit after it has been scaled turns out to be difficult.
Another method makes use of measuring variation in current flow obtained by applying a constant potential (of about -1 V relative to a saturated calomel reference electrode). Recording current variation provides information on the scaling of the electrode constituted by the metal under investigation. The apparatus containing the metal sample, the reference electrode, and the auxiliary electrode in water taken from the main circuit is itself placed in a thermostatically controlled bath. This method has the advantage over the preceding method of making it possible to forecast scaling, e.g. over a period of three hours at 40.degree. C. However, it suffers from the drawback that sensitivity cannot be changed without changing either the temperature or the imposed potential, since the same means are being used both for giving rise to scaling and for measuring it.
Further, since it is the bath itself which is heated or otherwise, rather than the metal sample, conditions on the surface of the sample are, by virtue of this very fact, very different from reality, in particular when considering heat exchangers. This means that the deposit is generally constituted by the calcite form of calcium carbonate, whereas in reality the aragonite form is obtained or else an association of both forms, depending on the temperature of the surface on which the deposit takes place.
Similar problems occur with phenomena of deposition or corrosion in the presence of liquids other than natural water.
In order to mitigate these drawbacks, the present invention seeks to provide a device enabling the conditions of the phenomenon to be created using parameters which are adjustable so as to reproduce the operating characteristics of the real circuit, or to create characteristics which accelerate the phenomenon, by using means for detecting the phenomenon and measuring variations therein, which means are separate from the means used for setting up experimental conditions.